The present invention relates to a process for forming holes by means of a focused ion beam (FIB) and, more particularly, to a method of effecting, at high speeds, ion beam machining which can form holes having good configurations, as well as an apparatus suitable for use with the method.
A typical prior art is described in J. Vac. Sci. Technol. B5(1), January/February 1987, pages 207-210 (hereinafter referred to as "document 1").
The document 1 proposes the following three points in connection with a case where a material 2 is sputtered by using such a focused ion beam 1 as shown in FIG. 2:
(1) If the relationship between a beam diameter d on the material 2 and a depth h of a hole which is formed in each step of irradiation with a focused ion beam is selected to be h&lt;d, then it is possible to reduce the amount of sputtered material redeposited on side walls of a hole which is being formed;
(2) If the direction of beam sweep is reversed after each pass, asymmetrical redeposition can be eliminated; and
(3) The redeposition onto the side walls can be compensated for by utilizing the dependency of a sputtering yield upon the incidence angle of a beam.
The proposal (1) takes account of the fact that, under the condition of h&gt;d, the size of a stepped portion of the hole which is being formed becomes large and, if the stepped portion is sputtered, a major part of the sputtered material tends to be scattered in oblique directions and redeposited on the side walls of the hole.
The proposal (2) is made in order to average the amount of obliquely scattered material by reversing the direction of beam sweep to make uniform the redeposition on the side walls of the hole, thereby keeping symmetrical the configuration of the hole which is being formed.
The proposal (3) is made on the basis of the relationship, shown in FIG. 3, between an incidence angle and a sputtering yield disclosed in "Sputtering Phenomenon" by Akira Kinbara, published by Tokyo Daigaku Shuppankai, Page 26 (hereinafter referred to as "document 2"). More specifically, the proposal (3) is based on a thought that since a beam is incident, at an equivalently large angle, upon the material redeposited on the side wall, the sputtering rate is high (the redeposited material is removed immediately).
The above-described prior art excels in that it clarifies the fact that formation of holes free from the redeposition of a large number of sputtered particles can be conducted under the processing 1 condition of h&lt;d. However, the prior art fails to take account of the necessity to realize a high-speed process under the condition of h&gt;d.